This invention relates to dynamoelectric machines such as electric motors and, more particularly, to an improvement for starting such motors when they are configured for low speed operation.
Certain household appliances, for example electric fans, are designed to operate at multiple speeds, with the operating speed at any one time selected by the user. An electric motor used to rotate the fan blade must be capable of running at, for example, a high speed, one or more medium speeds, and a low speed. (At a minimum, operation at a high speed and at a low speed is required for such appliances.) One problem with the multi-speed motors used in these appliances is they produce insufficient torque, when configured for low speed (rpm) operation, to start or "break away" the rotor. This problem is particularly serious when the motor uses sleeve bearings or bushings. One solution to the problem is to replace the sleeve bearings with ball bearings; however, ball bearings are more expensive than sleeve bearings and have increased noise transmission as compared to that of sleeve bearings. Since a fan, for example, is directly connected to the motor shaft, motor noise is more noticeable when ball bearings are used than with a motor employing sleeve bearings.
A second possible solution is to always start the appliance at one of the higher motor speeds and then switch the motor down to a lower operating speed. This approach, however, complicates the motor switching involved, is inconvenient to the user, and therefore is probably not acceptable to the user.
A third solution is to add certain electrical components to the motor circuit to increase the starting torque required of the motor at the lower operating speed switch settings. An example of this approach is shown in Italian Pat. No. 29291-A/77. In that patent, series connected resistors were connected in series to the parallel connected main and auxiliary motor windings. In addition, a positive temperature coefficient (PTC) device was connected in parallel across all three resistors. A multi-position electrical switch was set to increase or decrease the amount of resistance in the circuit to control motor speed. The PTC device shunted some of the current flow around the series connected resistors, during motor start-up, to improve starting torque. As the device then heated up, and its resistance increased, there was less current flow through the device. While this approach does improve low-speed starting torque, it unfortunately, is not the best solution because of continued power losses due to current flow through the resistors.